In many situations there is a need for monitoring a patient's respiration or to provide ventilations to a patient that is not breathing. Such situations might include cardiac arrest, respiratory obstruction, asthma, chronic obstructive pulmonary disease (COPD), heart failure, major trauma, overdose, seizure, sepsis and during anesthesia. In the following description the term “respiration” includes both spontaneous and assisted breathing/respiration as well as ventilations.
In some cases, patients with respiratory problems may be unstable and risk losing their respiratory function. Such patients need to be monitored in some way, either manually, for instance, by observing chest rise, or with the help of a respiration monitor system. However, such systems—commonly using a differential pressure flow sensor, or end tidal CO2 sensor—are often too expensive or not readily available in simple mask-and-oxygen scenarios commonly used out-of-hospital.
A common problem in situations where a rescuer will provide ventilations to a patient, e.g. during cardiac arrest, is poor adherence to guidelines for cardiopulmonary resuscitation (CPR). Often, the patient is hyperventilated which is deleterious to the patient. T. P. Aufderheide and K. G. Lurie, “Death by hyperventilation: a common and life-threatening problem during cardiopulmonary resuscitation,” Crit. Care Med, vol. 32, no. 9 Suppl, pp. S345-51, September 2004 and B. S. Abella, J. P. Alvarado, H. Myklebust, D. P. Edelson, A. Barry, N. O'Heam, T. L. Vanden Hoek, and L. B. Becker, “Quality of cardiopulmonary resuscitation during in-hospital cardiac arrest,” Jama, vol. 293, no. 3, pp. 305-10, Jan. 19, 2005 describe this problem. A solution to this problem is to have a device that detects ventilations and provides feedback on ventilation performance. Ventilations can, for instance, be detected by measuring the impedance change between defibrillator pads. However, impedance signals are often highly susceptible to noise (e.g. compression artifacts) and baseline drift problems requiring advanced and often expensive solutions. Also, the ventilation volume can not be measured using an impedance signal due to the variations of human physiology.
By help of flow sensors in the ventilation path, ventilation activity can be monitored and the ventilation volume assessed by integrating the flow. Flow sensors are typically based on the Venturi principle, i.e., the pressure drop associated with a flow restriction is measured. However, in order to achieve a high level of accuracy, this method requires a restriction with a very sophisticated geometry. This may often be forbiddingly expensive for a single-use unit. Single-use is normally desired or required for respiration measurements due to the risk of cross-contamination between patients.
The Venturi principle also has the disadvantage that two pressure outlets are needed from the restriction, which may complicate the geometry and increase cost. In addition, two pressure sensors are needed to measure the pressure drop in the restriction, which may increase cost. Alternatively, the two pressure sensors may be replaced with a single differential pressure sensor. If a differential pressure sensor is used, the output from the sensor can, however, only be used to assess flow, and not to monitor the absolute airway pressure, which may be important for detecting mask leakage, airway occlusion etc. Additionally, Venturi measurements are also known to be unstable in conditions of turbulent flow.
VentCheck™ by Respironics is a hand-held respiratory mechanics monitor that measures flow and pressure at the patient's airway. This monitor is designed for use on adults, pediatrics, and neonatal patients and can be used on any conventional ventilator. This monitor uses differential pressure single-use flow sensors to provide a breath-by-breath picture of the patient's respiratory status.
U.S. Pat. No. 6,203,502 describes a respiratory function monitoring device comprising a flow sensor and a conversion device. The device comprises two pressure transducers, one for measuring differential pressure corresponding to a gas flow rate, and a second to measure static airway pressure.
International Application WO 95/06234 describes a differential pressure sensor for measuring respiratory gas flow. The sensor is designed to have the capability of accommodating a wide variety of gas flow inlet conditions while employing a minimum of added system volume or resistance to flow.
U.S. Pat. No. 6,544,192 describes a patient monitoring apparatus for quantatively measuring a physiological characteristic of a patient. A first patient interface communicates with an airway of a patient such that substantially all gas inhaled and exhaled by the patient passes through the patient interface. One or more vent elements associated with the first patient interface communicate the first patient interface with an ambient atmosphere so that a pressure differential is created between the pressure in the first patient interface and the pressure of the ambient atmosphere. A sensor communicates with the first patient interface and measures a fluid characteristic resulting from this pressure differential and outputs a first signal indicative of that characteristic.
There is, therefore, a need for a simple and inexpensive device and method for monitoring respiration/ventilation of a person to overcome the above mentioned problems associated with the prior art.